Curbs on emissions of carbon dioxide and other substances have been strengthened against a background of a growing environmental protection movement, and in the automobile world there is now vigorous development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) alongside vehicles using fossil fuels such as gasoline, diesel oil and natural gas. In addition, the soaring price of fossil fuels in recent years has acted to spur on the development of EVs, HEVs and the like.
The batteries used for such EVs, HEVs and the like are generally nickel-hydrogen secondary batteries or lithium ion secondary batteries. However, what is now required of such vehicles is not only environmental friendliness, but also basic performance as an automobile, in other words, superior driving capabilities. Therefore, it is necessary not simply to enlarge the battery capacity, but also to increase the battery output, which exerts large effects on an automobile's acceleration and hill-climbing performance. However, when a high output is discharged, a large current flows in the battery, and as a result there is a large increase in heat due to contact resistance between the substrates and the collectors, which are the generation elements. Thus, batteries for EVs and HEVs are required not only to be large-sized and large capacity, but also to handle a large current. Accordingly, in order to prevent electricity loss inside the battery and thereby reduce heat emission, many improvements have been carried out with regard to lowering the internal resistance by preventing welding faults between the substrates and collectors, which are the generation elements.
There exist the methods of mechanical caulking, welding and the like for electrically joining the substrates and collectors, which are the generation elements. Welding, which is joining by fusion, is appropriate as the electrical collection method for batteries of which high output is required. Also, in order to effect low resistance, the material used for the negative electrode assembly of a lithium ion secondary battery is copper or copper alloy, which however have the characteristics of low electrical resistance and high thermal conductivity, so that an extremely large amount of energy is required in order to weld them.
The following methods have long been known as methods for welding together the substrates and collectors which are the generation elements:
1) Laser welding (see JP-A-2001-160387)
2) Ultrasonic welding (see JP-A-2007-053002)
3) Resistance welding (see JP-A-2006-310254)
With the laser welding method, a high-energy laser beam is required because the reflectivity of the copper or copper alloy welded material with respect to the YAG (yttrium-aluminum garnet) laser light that is widely used to weld metals is high—around 90%. There also exist the problems that when copper or copper alloy is laser-welded, the weldability varies greatly depending on the condition of the surfaces, and that the occurrence of spattering is unavoidable, as in laser welding of other materials.
Ultrasonic welding also requires a large amount of energy, because the thermal conductivity of the copper or copper alloy welded material is high. Also, the negative electrode activate material may be dislodged by the ultrasonic vibration during welding. Accordingly, in the invention disclosed in JP-A-2007-053002, the electrode assembly, which is the generation element, is compressed during ultrasonic welding, so that dislodged negative electrode active material will not enter inside it.
Further, with resistance welding, due to the copper or copper alloy welded material having low electrical resistance and high thermal conductivity there exist the problems that large current needs to be input in a short time, that fusion-joining of the collectors and the electrode poles, which are of the same material as the collectors, sometimes occurs during welding, and that melting or spark generation may occur at places other than the welds.
Thus, the three welding methods have their merits and drawbacks. In the interests of productivity and economy however, the resistance welding method, which has long been used as a method for welding between metals, will preferably be employed. However, especially in order to resistance-weld the copper collectors to the substrates of copper or copper alloy in the electrode assembly (see JP-A-2002-008708) of sealed batteries for EV and HEV application, which have exposed portions of positive electrode substrates at one end and of negative electrode substrates at the other, a great deal of welding energy is necessary in order to effect a firm weld, since the electrode assembly has a large number of stacked layers. Moreover, when the welding energy is rendered large for resistance welding, the generation of spattered particles is increased and there is increased such a probability that the particles move into the inside of the electrode assembly, so that the internal short circuit is caused.